The present invention relates generally to battery charging circuitry and, more particularly, to a self-adapting voltage amplifier and battery charger detection.
Many portable devices today have battery charger circuitry that connects to a power source via a Universal Serial Bus (USB) connector. USB is an industry standard that defines the cables, connectors and communications protocols used by a bus for connection, data communication and power supply between electronic devices and apparatus. A USB portable device includes a USB receptacle (female) connector and typically includes a battery. The USB receptacle has power supply pins for power supply, and data communication D+ and D− pins, and may also include an identification (ID) pin. A USB battery charger includes a USB plug (male) connector, and is capable of supplying battery charging current to a portable device through the power supply pins of the USB connector. The battery charger also may be capable of data exchange with the portable device through the D+ and D− pins of the USB connector.
The USB battery charging specification defines various kinds of upstream chargers that may be connected to a downstream portable device through a charging port to supply power to the portable device battery. A charging downstream port (CDP), on an upstream apparatus such as a computer, supports data transfers as well as power supply. A dedicated charging port (DCP) supplies power but does not support data transfer and may be a converter that converts power from a mains supply or from a vehicle's power supply to a USB charging power supply. An accessory charger adapter (ACA) is an adapter that allows a single USB port to be attached to both a charger and another USB device (for data transfer) at the same time. An ACA-dock is a docking station that has an upstream port and may or may not have downstream ports. A portable device can recognize the type of USB port from the way the D+ and D− pins are connected in the battery charger and the voltages on them.
The USB Battery Charging specification revision 1.1 specifies that, for mutual detection of the type of battery charger and the type of portable device connected together through the USB connector, the USB portable device should maintain a voltage VDP—SRC 0.5-0.7V on the D+ pin during charger detection while sourcing at least 250 μA current. This is the only requirement during battery charger detection for the D+ voltage source of the portable device in the USB battery charging specification rev 1.1. A conventional voltage follower operational amplifier, often included in a portable device as a buffer amplifier, with its output connected to the D+ pin for data signal communications, can meet this requirement.
In order to support the detection of an ACA-Dock, the USB battery charging specification revision 1.2 also requires that the voltage source output of the portable device shall not pull the D+ pin below 2.2V through a 0.9-1.575 kΩ pull up resistance RDP—UP that is connected to a voltage VDP—UP between 3.0V and 3.6V in the battery charger. A conventional voltage follower cannot meet this requirement due to its negative feedback circuit. As the input of the buffer amplifier is always 0.6V, its output tries to pull the D+ pin down to 0.6V while the pull up resistor RDP—UP in the battery charger is trying to pull the voltage of the D+ pin up. It would be desirable to have a portable device capable of meeting this requirement of the USB battery charging specification revision 1.2.